he structure and the expression of the genes for human class I antigens of the major histocompatibility complex have been tudied at the molecular level. The nucleotide sequence etermined for a cDNA clone (derived from mRNA) and for part of genomic DNA clone (derived from cellular DNA), both encoding he beta chain of an SB antigen, has defined the primary tructure of an SB beta chain. It was possible for the first ime to determine the evolutionary relationship of SB beta hains to other beta chains in the human HLA-D region and in he murine H-2 I-region. The beta chain genes of DR (I-E quivalent), DC (I-A equivalent) and SB (no murine equivalent) ntigens have diverged from each other to approximately the ame extent. In order to analyze further the multiple onallelic HLA class II genes a library of cDNA clones has been onstructed from mRNA of an HLA-hemizygous mutant B cell line. everal vectors have been constructed that will permit the xpression of inserted cDNA sequences after transfection into ukaryotic cells. The availability of multiple human class II enes which can be expressed will lead to a dissection of their espective role in alloreactions and antigen presentation to T ells.